This invention relates generally to apparatus for segregating relatively dense paramagnetic particles from relatively light diamagnetic particles, and more particularly to magnetic devices for separating particles having a high mineral and pyritic sulfur content from particles of organic coal in order to reduce the overall mineral and sulfur content of the coal.
Treatment of coal to remove sulfur content is well established commercially for both metallurgical and steam generation markets. The most widely used systems employ liquid mediums to separate organic coal from mineral and iron pyrite inclusions. Such conventional systems are operable in part because the mineral and iron pyrite inclusions are generally denser than the organic coal. However, the use of conventional coal cleaning processes in cleaning fine size coals is not economical. This is especially true of fine coal fractions that have been cleaned successfully into a relatively low ash product, but have poor thermal recoveries because of the surface moistures that cling to fine coal. The requirements for separating, handling, and dewatering fine sized coal are so expensive that a significant fraction is usually rejected to the waste pond. Processes such as magnetic beneficiation that are operated on a dry rather than a wet coal feed are therefore preferable.
It has been observed experimentally that when pulverized coal passes freely through a magnetic field gradient, the mineral components tend to separate from the organic components because of the different inherent magnetic characteristics of the mineral and organic components of the coal. The inorganic iron pyrites (iron disulfide) and ash producing components are paramagnetic in part because of the inclusion of trace concentrations of monoclinic iron pyrrohotite. In contrast, the organic components (including the "clean coal") are diamagnetic.
A paramagnetic particle becomes slightly magnetized in the presence of a magnetic field so that, if the field is non-uniform, the particle will be drawn toward the region of higher field intensity. A diamagnetic particle behaves exactly the opposite, and tends to move in the direction of lower field intensity. Open gradient magnetic separation (OGMS) takes advantage of these characteristics to separate the organic fraction of the coal from the inorganic ash and pyritic fraction. A further discussion on the physical basis for this separation is to be found in R. D. Doctor, C. B. Panchal, C. Swietlik, "The Development of Open Gradient Magnetic Separation for Coal Cleaning Using a Superconducting Quadropole Field," Paper 48e, AICHE National Meeting (1985).
Recent interest in magnetic coal beneficiation techniques has focused on the application of high gradient magnetic separation (HGMS), which has been used commercially for beneficiation of mineral ores. In HGMS, a grid of ferromagnetic filaments are placed in a uniform magnetic field. The necessary high gradients are induced locally around the ferromagnetic filaments, and paramagnetic minerals are trapped on the filaments. When the filaments are moved out of the magnetic field the particles will fall off. Thus, HGMS is useful in intermittent or batch type processes.
The use of superconducting magnetic devices for the beneficiation of coal was suggested in the work entitled "Initial Exploration of Application of Open Gradient Magnetic Separation of Coal to Beneficiation of Liquification Feeds" by E. C. Hise, Oak Ridge, Tenn., ORNL/TM8529 published February, 1983. However, the system of Hise describes coal dropping through an open gradient field with the pyritic material and ash tending to become attached to the wall of the bore through which the coal is passing. While the Hise system operates effectively at low flow rates, it experiences significant difficulties in overcoming particle-particle interaction when large numbers of particles are present. Hence, it is of limited suitability for many commercial applications.
Screw-type separators have been used in magnetic separation systems to remove foreign particles from oil and other feed materials. However, such separators also have not been useful in systems which employ high flow rates. Thus, there is a need for improved apparatus and methods for segregating paramagnetic particles from diamagnetic particles, particularly in coal, at a relatively high flow rate. There is also a need for improved apparatus and methods for segregating paramagnetic particles from diamagnetic particles which are suitable for commercial applications where large numbers of particles are present.
Accordingly, an object of the present invention is to provide new and improved apparatus for segregating paramagnetic particulate material from diamagnetic particulate material.
Another object of this invention is to provide new and improved high speed, continuous, low cost methods and apparatus for separation of paramagnetic and diamagnetic particles in general, and in particular, for separation of ash and pyrite from coal, so as to substantially reduce the sulfur content of the coal.
It is an additional object of the invention to provide new and improved apparatus that combines forces of vibration, gravity and magnetic flux to separate paramagnetic particles from diamagnetic particles.